Herbivore species richness and feeding complementarity affect community structure and function on a coral reef

TLDR

Consumer effects on prey cascade through food webs, and bottom‑up effects of primary producer diversity are known, yet the influence of consumer diversity on community structure and ecosystem function remains unclear. Over two years, the authors used large cages to compare single‑species herbivores, mixed‑species herbivores, and herbivore exclusion, measuring impacts on seaweed and coral communities. Mixed‑species herbivore treatments reduced macroalgae by 54–76%, increased crustose coralline algae by 52–64%, boosted coral cover by 22%, and prevented coral mortality, demonstrating that complementary feeding by diverse herbivores sustains coral reef structure and function.

Abstract

Consumer effects on prey are well known for cascading through food webs and producing dramatic top-down effects on community structure and ecosystem function. Bottom-up effects of prey (primary producer) biodiversity are also well known. However, the role of consumer diversity in affecting community structure or ecosystem function is not well understood. Here, we show that herbivore species richness can be critical for maintaining the structure and function of coral reefs. In two experiments over 2 years, we constructed large cages enclosing single herbivore species, equal densities of mixed species of herbivores, or excluding herbivores and assessed effects on both seaweeds and corals. When compared with single-herbivore treatments, mixed-herbivore treatments lowered macroalgal abundance by 54–76%, enhanced cover of crustose coralline algae (preferred recruitment sites for corals) by 52–64%, increased coral cover by 22%, and prevented coral mortality. Complementary feeding by herbivorous fishes drove the herbivore richness effects, because macroalgae were unable to effectively deter fishes with different feeding strategies. Maintaining herbivore species richness appears critical for preserving coral reefs, because complementary feeding by diverse herbivores produces positive, but indirect, effects on corals, the foundation species for the ecosystem.